Centering and blocking devices are appliances in widespread use in the field of optics. They are used in the process of fabricating a pair of spectacles, immediately prior to machining the ophthalmic lenses in order to fit them to the shape of the spectacles frame that has been selected.
Usually, a “raw” ophthalmic lens is substantially circular in shape, being of diameter that is sufficient to enable it to be mounted properly in the rim of the selected frame.
A centering and blocking device is then used to fix a handling peg on the ophthalmic lens in question, known as a “block”.
In a subsequent fabrication step, the handling peg is used for rotating the ophthalmic lens in order to machine it.
The handling peg is put into place on the front face of the ophthalmic lens at a point that is determined by calculation as a function in particular of the position of the “optical center” (in the broad sense of the term) or more generally the center point of the lens, the shape of the selected frame, and certain characteristics of the wearer, in particular the pupillary distance or half-distance and the height of the frame (the height of the pupils relative to the bottom portions of the rims of the frame).
Whether in automatic mode or in manual mode, most previously-known centering and blocking devices detect the position of the optical center or of the center and/or axis markings of an ophthalmic lens by illuminating said lens by means of a light beam and by sensing the light beam that has passed through the lens. In the resulting image, the devices identify the shadows of the center and/or axis markings.
Such devices commit an error in detecting the position of the center markings (typically the mounting cross or the marking points that result from centering on a frontofocometer) and/or the axis markings (horizontal lines) of the ophthalmic lens. This error is the result of prismatic deflections of the shadows of the markings, induced by the lens itself, and depending on the spherical, cylindrical, and prismatic optical powers of the ophthalmic lens in the neighborhood of the marking in question.
For example, if the ophthalmic lens for centering presents lateral prismatic power in the neighborhood of the marking in question, the shadow of the marking in the image will appear offset laterally relative to the real position of the marking on the front face of said lens in a direction and by an amount corresponding to the angle of the prism.
Similarly, if the ophthalmic lens presents toroidal power, such centering and blocking devices can commit an error in detecting the axis marking if the axis formed by the markings and the main axis of the corresponding torus are not parallel or mutually perpendicular.
Furthermore, in automatic mode or in manual mode, the image observed by the signal acquisition means of prior art devices is displayed in real time on the display screen of the device for viewing by the operator.
In automatic mode, the operator can thus monitor and confirm the steps in the centering operation. In manual mode, the operator moves the ophthalmic lens by hand so as to bring the marking of said lens into coincidence with a centering sight encrusted in the displayed image.
Thus, any calculation for correcting the above-mentioned detection error needs to be performed in real time, which imposes short response times from the calculation equipment in order to ensure that the display is fluid. Nevertheless, the calculation hardware cannot make use of technology that is too expensive, given the market price for centering and blocking devices of this type.
In an attempt to remedy this problem of error in detecting the centering marking of a lens, document EP 0 409 760 proposes a centering and blocking device in which, firstly the light path of the light for detecting the position of the optical center or of the center and/or axis markings of the lens is reversed, i.e. the ophthalmic lens is illuminated from behind (given that the center and/or axis markings are provided on the front face of the lens) and the light that has passed through the lens is sensed from beside its front face, and secondly the translucent screen for sensing said light flux and disposed in front of the acquisition means is disposed as close as possible to the front face of the lens for centering so as to limit the distance traveled by the deflected light rays prior to being focused on the acquisition means.
Nevertheless, that requires the translucent screen to be mounted in movable manner on the structure of the device so as to allow it to be retracted in order to enable the handling peg to be deposited on the determined location of the front face of the ophthalmic lens.
That complex mounting of the screen on the structure of the device increases the size of the device, its manufacturing cost, and above all does not ensure that measurement accuracy is long-lasting.